Eight of twelve serologically different lipooligosaccharides (LOS) of Neisseria meningitidis bound a mouse monoclonal antibody (anti-My-28) which recognizes lacto-N-neotetraose (LNnT), Galbeta1--4GlcNAcbeta1--3Galbeta1-- 4Glc. Among the twelve LOS immunotypes, types 2, 3, 4, 7, 8, and 9 exhibited strong binding; types 5 and 10 were moderate; and types 1, 6, 11, and 12 were negative as measured by ELISA, immunodot and immunoblot assays. If an LOS showed multiple components by SDS-PAGE analysis, the antibody re- active epitope was expressed on the larger major component. The expression of the reactive epitope on the LOS was influenced by growth medium and the epitope could be masked by sialylation when N. meningitis was grown in tryptic soy broth. N-Acetyllactosamine inhibited the binding of the antibody to all eight reactive LOS. The antibody binding to a represntative LOS (type 2) was best inhibited by LNnT, next by N-acetyllac- tosamine, but not inhibited by lacto-N-tetraose, Galbeta1--3GlcNAcbeta1-- 3Galbeta1--4Glc. These results suggest that the LNnT sequence is present in 8 of 12 immunotype LOS. The presence of the LNnT sequence, a structure expressed by a variety of human cells, in the LOS may play a role in the virulence of N. meningitidis by enabling the organism to evade host immune defenses.